This invention relates to an image forming apparatus such as a copier, a page printer, a facsimile machine, an MFP (i.e., Multifunction Peripheral) or the like, and relates to an image forming unit used in the image forming apparatus to form an image by developing a latent image with developer (for example, toner).
The image forming apparatus using electrophotography includes an image forming unit. The image forming unit includes a photosensitive drum as an image bearing body, a charging roller that uniformly charges the surface of the photosensitive drum, an exposing unit that exposes the surface of the photosensitive drum to form a latent image thereon, a developing roller (i.e., a developer bearing body) that develops the latent image on the photosensitive drum with toner, and a cleaning device that removes the residual toner from the surface of the photosensitive drum. The image forming unit can be configured as a part of a main body of the image forming apparatus, or can be configured as a process cartridge which is detachable from the main body of the image forming apparatus.
Generally, in the image forming unit, the developing roller is disposed in contact with the photosensitive drum or disposed facing the photosensitive drum with a small gap, so as to cause the toner to adhere to the latent image on the photosensitive drum. The developing roller is driven by a driving mechanism to rotate.
In order to adjust a gap between the developing roller and the photosensitive drum or a contact pressure between the developing roller and the photosensitive drum, it is necessary to adjust a center-to-center distance between the developing roller and the photosensitive drum. Therefore, a recently proposed image forming apparatus uses eccentric cams supporting a shaft of the developing roller via bearings. The eccentric cams are rotatably inserted into insertion holes formed on side plates of a chassis of the image forming unit. By rotationally adjusting the eccentric cams in the insertion holes, the position of the shaft of the developing roller changes, and therefore the center-to-center distance between the developing roller and the photosensitive drum changes. Such an image forming apparatus is disclosed by, for example, Japanese Laid-open Patent Publication No. 2006-48018 (see, paragraphs 0068 to 0069 and FIG. 12).
However, in the above described image forming apparatus, gaps may be formed between the eccentric cams and the insertion holes. In such a case, after the center-to-center distance between the developing roller and the photosensitive drum is adjusted, the center-to-center distance between the developing roller and the photosensitive drum may change due to the above described gap when the developing roller and the photosensitive drum rotate. Therefore, there is a possibility that a part of the latent image (on the photosensitive drum) may not be developed, with the result that white spots may appear in a printed image.
In order not to form a gap between the eccentric cam and the insertion hole, it is considered to make the outer diameter of the eccentric cam larger than the inner diameter of the insertion hole. However, in this case, the eccentric cam must be press-fit into the insertion hole, and therefore the rotational adjustment may become difficult because of high rotational friction.
In order to solve these problems, it is necessary to maintain strict dimension tolerances of the eccentric cam and the insertion hole, and therefore difficulties in manufacturing and controlling of the components may increase.